In recent years there has come about a requirement for flash or arc lamps to operate in a mode known as "pulse and simmer" for some laser pumping applications. Such lamps consist typically of a cylindrical transparent body fitted with a metal electrode at each end. During manufacture the lamps are filled with a gas or gas mixture before final sealing.
In the pulse and simmer mode a trigger pulse is used to initiate electrical breakdown between the electrodes within the lamp. However, thereafter the plasma can be maintained as a continuous streamer within the lamp by a much lower DC voltage. This continuous DC level maintains the lamp in a "ready" condition with no further need for triggering, and is known as the simmer condition--the current flowing during this condition being referred to as the simmer current.
The lamp is then made to flash at intervals, as required by large voltage pulses, usually applied to the lamp by a separate circuit. Diodes are used to prevent the interaction of the trigger, simmer and main pulse circuits other than as described above.
Typical simmer currents are between 10mA and 5A. For a lamp with a bore of 8mm or so, a mean power level of up to 10 kW per 200mm of arc length would be expected.
In use the main pulse frequency may vary from 0.1Hz to 100 Hz. The main pulse period might then vary from some tens of milliseconds down to parts of a millisecond respectively. Main pulse currents for an 8mm bore lamp might be over 1000A for very short pulses as required in some drilling applications, or around 150A for some longer pulse welding applications, where a lower repetition rate is usually expected.
It is well known to users of conventional pulse and simmer arc lamps that, given the same mean power level, low current long period operation with long intervals between pulses results in faster failure of the lamp than high current short period high repetition rate operation. Thus laser drilling tends to give long life, whereas laser welding results in early lamp failure.
The normal failure mode of lamps of conventional construction is the obscuring of the transparent envelope by deposits from the lamp cathode. This process eventually results in the laser output dropping unacceptably. Conventional lamp cathodes are made by tipping a thoriated tungsten mount with a porous tungsten cathode impregnated with, for example Barium Calcium Aluminate. The whole structure is normally cylindrical and of a similar diameter to the lamp bore, the tipped working face generally being almost flat. The components of the electrodes are usually assembled by brazing or welding.
The tip of the cathode is very familiar as the working element of many high power vacuum valves and tubes, where loadings up to 100 A/cm.sup.2 and cathode temperatures of 1500.degree. C. might be achieved. In conventional flash lamps loadings of over 1000 A/cm.sup.2 can be demanded, and in such lamps the cathode has therefore been cooled.
Degradation of the cathode has been thought to be caused by shock to and overheating of the low work function surface of the cathode. Heavy cracking of the impregnated cathode tip has usually been visible after just a few minutes operation in any high power mode, but damage is generally most severe after high power welding. Molten beads of Tungsten are apparent on the tip surface, and beads of molten metal may be visible in the bore of the lamp.
As a result, most attempts to improve these cathodes have concentrated on improved cooling, higher strength, and improved shock resistance.
Whilst the present invention has arisen in answer to a specific need for an improved pulse and simmer cathode it is believed that the improvement is equally applicable to standard pulse lamps operating with no simmer supply.